Next the DSLR is going to slip away into the sands of time, as they are replaced by the smaller, lighter, and more elegant mirrorless camera.

I don't think the DSLR is going to "slip away into the sands of time." It'll just become more of a niche product. But we're still a long way from that. There will still be people who favor an optical viewfinder, just like there are people who still favor using a rangefinder. Keep in mind that, even with the massive success of SLRs, rangefinders still haven't "slipped away into the sands of time."

Well, they pretty well have. Around 1970, a fixed-lens rangefinder was the commonest moderately priced camera, selling to the market that today buys a good quality fixed-lens zoom camera. Canon must have sold many millions of their various Canonet models, for instance.

Now the Leica is the only survivor, selling in very small numbers.

Not the best analogy, my friend. First of all, rangefinders never had as much market penetration and popularity as SLRs, and certainly never had as much ubiquity as today's DSLRs. Furthermore, a "fixed-lens rangefinder" like a Canonet certainly does not have the same lock-in holding power as any ILC (interchangeable lens) camera SYSTEM in which a company-- and its user-- are FAR more heavily invested in various parts of the system. A "fixed-lens rangefinder" is simply a stand-alone product like a point-and-shoot camera, and it's very easy to leave it. Not so with system cameras. And obviously, a "fixed-lens rangefinder" is also a lot more limiting and a lot less flexible than system cameras for which countless lenses and accessories exist. Again, making it very easy to leave. So a "fixed-lens rangefinder", even a "commonest" one, is hardly a good analog to today's DSLR cameras and the lens systems that support them. Your Canonet example is more akin to a compact P&S camera-- limiting and practically disposable. Sure, it might have been "commonest", but people ditch stand-alone compacts like they change shirts.

Around 1970, the "commonest" moderately priced *ILC* camera was a Canon AE-1...an SLR, not a rangefinder. The AE-1, alone, outsold the most popular rangefinders many times over. It, alone, probably outsold all rangefinders combined. SLRs, once they came into existence, were far far far more popular than rangefinders, which was a lopsided success that continues even to this day. So not only are SLRs far more popular and successful than rangefinders ever were, they are also superior in design, function, and flexibilty than rangefinders ever were (e.g., rangefinders can't even do AF). So to say that DSLRs will slip to rangefinder-level popularity is extremely far-fetched because rangefinders were never that popular to begin with...and your Canonet example is simply an erroneous example because it was simply a fixed-lens compact P&S.